Spray coating systems are well known in the art. Such systems ordinarily include a conveyor carrying a series of articles to be coated along a path which, in part, passes through the spray chamber of a spray cabinet or booth. Located in the spray chamber are spray nozzles which spray a coating on each of the articles as they pass by.
There has recently been developed by the assignee of this invention a process for providing a substrate such as polyethylene terephthalate (PET) bottles with a gas barrier coating of a copolymer of vinylidene chloride. This barrier coating prolongs the shelf life of product contained in the bottle by retarding carbon dioxide migration through the walls of the bottle, oxygen penetration into the bottle, and water migration and penetration. For example, the process is particularly applicable to provide PET bottles with a barrier coating to prevent the loss of carbonation of beverages contained therein.
The process involves locating each container to be coated in close proximity to one or more airless spray nozzles, and impacting the outside surface of the container with a stream of stabilized aqueous polymer dispersion, such as an aqueous polyvinylidene dispersion, to obtain a uniform coating on the surface of the container. It has been found that the impacting force of the spray on the container quite unexpectedly leads to barrier coatings which exceed known properties heretofore achieved by the industry. It has been found critical to obtaining a uniform film of polymer having superior adhesive and barrier properties that the spray coating reach the substrate surface with a force sufficient to cause phase inversion on the surface and not before. This requires positioning of the spray nozzles in close proximity to the bottles to be coated such that the outside surface of the container is impacted with the stream of coating material. It is also important that the spray coating impact the surface of the bottle over its entire surface to provide a uniform, continuous barrier coating.